1. Field of the Invention
The present invention relates to an infrared sensor calibration apparatus used for calibrating an infrared sensor loaded into a space navigating object such as an artificial satellite.
2. Description of the Related Art
It is well-known that a blackbody is employed in an apparatus for calibrating an infrared sensor.
The strength of infrared rays radiating from the surface of the blackbody univocally depends upon the temperature and the emissivity of the surface by the Planck radiation law. Using this law, the infrared sensor measures the strength of the infrared rays radiating from the blackbody having a known temperature thereby to perform calibration, i.e., so-called graduation on the basis of the relationship between the voltage output from the infrared sensor and the temperature of the blackbody.
FIG. 3 shows a prior art apparatus for calibrating an infrared sensor 1 as described above. In this apparatus, a blackbody 2 is provided opposite to an incidence entry 1a of the sensor 1. A heating/cooling system 5 is buried into the blackbody 2. The system 5 is driven by a temperature controller 3 in response to a command signal and set to have a predetermined temperature. The infrared rays whose intensity are determined by the temperature of the system radiates from the blackbody 2 toward the incidence entry 1a.
The output of the infrared sensor 1 is connected to a data processing unit 4. The processing unit 4 processes data of voltage generated from a detector of the sensor 1. The output of a temperature detector 6 for detecting the temperature of the blackbody 2 is also connected to the data processing unit 4. The processing unit 4 is supplied with data of the temperature of the surface of the blackbody 2. Thus, the data processing unit 4 is so constructed that it compares the voltage data obtained from the infrared sensor 1 and the temperature data of the blackbody 2 to calibrate the infrared sensor.
Generally, the relationship between emissivity .epsilon. and reflectance r of the surface of an object is expressed by the following equation, assuming that the surface of the object does not absorb any infrared rays (absorptance .alpha.=0). EQU .epsilon.+r=1 (1)
Using the Planck's radiation law, the radiant intensity L of infrared rays radiating from the surface of an object having a temperature of T1, can be given by the following. EQU L=.epsilon.L (T1) (2)
In actuality, however, since the emissivity .epsilon. cannot be "1", the object has some reflectance r. Let us consider a model in which an object 7 is covered with an atmosphere 8 of temperature T2, as shown in FIG. 4. The effective radiant intensity L of this model is expressed as follows. EQU L=.epsilon.L(T1)+rL(T2) (3)
In the foregoing prior art infrared sensor calibration apparatus, it is desirable that the emissivity .epsilon. of the radiating surface of the blackbody 2 should be "1" in order to calibrate the infrared sensor with high precision. If .epsilon.=1, r becomes zero from the equation (1) and thus the second term of the equation (3) need not be taken into consideration, with the result that the radiant intensity of the blackbody 2 has only to be evaluated by the equation (2).
In the prior art apparatus described above, however, the emissivity .epsilon. of the radiation surface of the blackbody 2 is 0.8 to 0.9 and thus the reflectance r is 0.1 to 0.2. Since, as shown in FIG. 5, infrared rays B radiated from the environment and then reflected by the blackbody 2 and infrared rays C radiated directly from the environment as well as infrared rays A radiated from the blackbody 2, are incident upon the entry 1a of the sensor 1, these infrared rays B and C will become errors, in other words, an error will occur in the second term of the equation (3).
The intensity of infrared rays A can be easily obtained from the temperature of the blackbody 2, but that of infrared rays B or C cannot be correctly done in actuality. Therefore, the prior art infrared sensor calibration apparatus has the drawbacks wherein it is difficult to eliminate the errors of the infrared rays B and C and also difficult to compare the output voltage of the infrared sensor 1 and the temperature of the blackbody 2 with high precision.